Air/fuel ratio sensor for air/fuel ratios in excess of stoichiometry

ABSTRACT

The present invention provides a sensor which is operative to sense the partial pressure of oxygen in the exhaust gases of combustion processes having air/fuel combustion mixture ratios in excess of the stoichiometric ratio. The sensor according to the present invention relies upon the direct and generally linear change in the logarithm of the resistance of cobalt monoxide ceramic material as a function of the partial pressure of oxygen in the environment of the sensor element at elevated temperatures for partial pressures of oxygen which coincide with the partial pressures of oxygen in the exhaust gases of fuel-lean combustion mixtures. The present invention provides a generally cylindrical cobalt monoxide ceramic sensor element, supported within a housing formed of a compatible ceramic material such as alumina, and arranged so that the change in the electrical resistance between the ends of the cylinder of the sensing element may be measured. When the sensor is situated within the exhaust system of an internal combustion engine, the change in electrical resistance is a measure of the partial pressure of oxygen within the exhaust system and is also indicative of the air/fuel ratio of the combustion mixture which has produced the exhaust gases of the environment of the sensor. Such indication is usable to maintain the combustion mixture at a desired air/fuel ratio.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the field of internal combustionengine analyzers. More particularly, the present invention is directedto that portion of the above-noted field which is concerned with theanalysis of the air/fuel ratio of the combustion mixture provided to theengine. More particularly still, the present invention is directed tothat portion of the above-noted field in which analysis of the chemistryof the exhaust gases produced by the engine results in a determinationof the air/fuel ratio of the combustion mixture being provided to theengine. With greater particularity still, the present invention isdirected to that portion of the above-noted field which is concernedwith the provision of a sensor for accurately sensing the air/fuel ratioof a combustion mixture by changing an electrical characteristic inresponse to changes in the partial pressure of oxygen present in theexhaust gases resulting from the combustion process where the air/fuelratio for the combustion mixture is to be maintained at a value inexcess of the stoichiometric value.

2. Description of the Prior Art

The need for analyzing the operation of an internal combustion engine toimprove or maintain the efficient operation thereof is well recognized.It has been suggested, at least as early as 1937, that the air-to-fuelratio of a combustible mixture provided to an internal combustion enginemay be determined by analyzing the composition of the exhaust gasesproduced from combustion of the mixture. An example of such a systemappears in U.S. Pat. No. 2,077,538 -- "Exhaust Gas Analyzer ForAutomotive Vehicles" issued in the name of L. S. Wait. According to thesystem disclosed in the Wait patent, a variable resistance element issituated in the exhaust stream of an internal combustion engine andarranged within an electrical bridge network, such as for example aWheatstone bridge, so that the heat dissipative qualities of thevariable resistance element will respond to changes in the chemistry ofthe exhaust gases to provide an indication of the air/fuel ratio of thecombustion mixture being provided to the engine.

More recently, the efforts to reduce the pollution of our atmospherehave prompted implementation of stringent controls upon an automotiveengine exhaust emissions. In efforts to reduce the amount of pollutantsinjected into our atmosphere by mobile internal combustion engines, ithas become apparent that accurate and self-adjusting control of theair/fuel ratio of the combustion mixture provided to such engines isdesirable. For example, copending commonly assigned U.S. Pat. No3,868,846, issued on Mar. 4, 1975 in the names of T. Kushida et al. andtitled "Circuit For Converting A Temperature Dependent Input Signal To ATemperature Independent Output Signal" discloses one system wherein atitania exhaust gas sensor is responsive to changes in the exhaust gaschemistry, caused by excursions of the air/fuel ratio of the combustionmixture being provided to the engine from a selected value at, orricher, in fuel than, the stoichiometric value, to automatically andreliably control a controller means to maintain the air/fuel ratio ofthe combustible mixture at the selected value. It is also known thatzirconia-based sensors may be similarly used. Other materials for use asexhaust gas sensor elements responsive to departures of a combustionmixture from a fuel rich value to a fuel lean value, and vice versa, bypassing through the stoichiometric value are known.

The exhaust gases produced by fuel-rich combustion form an atmospherewhich may be viewed as a reducing atmosphere. Many materials are knownto respond electrically to changes in the chemistry of a reducingatmosphere. Exhaust gas sensors responsive to departure of a combustionmixture from the stoichiometric value to a fuel-rich or fuel-leancondition to produce a large scale change in an electricalcharacteristic are also known. Some of the reducing gas sensingmaterials also demonstrate this change which may ideally represent astep-function change. An example of this phenomenon is discussed incopending, commonly assigned U.S. Pat. No. 3,886,785, issued June 3,1975 in the names of H. L. Stadler et al. The value of thestoichiometric responsive materials resides in this large scaleexcursion of an electrical parameter which may be utilized to directlyand rapidly sense departure of the air/fuel ratio of the combustionmixture from stoichiometry.

It has become apparent that operation of an internal combustion enginewith a combustion mixture at the stoichiometric value results in acombination of gas pollutants which combination is expensive anddifficult to correct by means of exhaust gas reactors. For example,combinations of two or more such reactors are known to be required toeffectively reduce the exhaust gas pollutants produced under suchconditions to values which the Environmental Protection Agency of theUnited States Federal Government asserts to be an environmentally safein internal combustion engine exhaust. While operation of the engineunder either fuel-rich or fuel-lean conditions can overcome thisdifficulty, the presently available exhaust gas sensors function underfuel-rich conditions and the resulting excess consumption of fuel ismost undesirable. Operation of the engine under fuel-lean conditions istherefore preferrable since the wasteful excess consumption can beavoided. The exhaust gases produced under such a situation constitute anoxidizing atmosphere and it is therefore a specific object of thepresent invention to provide a sensor capable of operation in anoxidizing environment to function as an air/fuel ratio sensor.

The operation of an internal combustion engine in the "lean regime,"(that is, with an air/fuel ratio of from about 15 to about 22) andparticularly at high values of the air/fuel ratio results in automaticlessening of the major exhaust gas pollutants (CO, HC and NO_(x)).Removal of residual amounts of any of the major pollutants is thereforerelatively simple. This relative simplicity of operation is predicatedupon maintenance of the air/fuel ratio at a selected and preciselycontrolled value. Heretofore, reliable operation of the internalcombustion engine within the lean regime at any one selected air/fuelratio has been made virtually impossible due to the lack of a suitablesensor which could respond to variations in the air/fuel ratio from itsselected value within the lean regime. It is therefore a specific objectof the present invention to provide a sensor which is operative forcombustion mixtures within the lean regime and which may be used toaccurately and reliably monitor the air/fuel ratio of the combustionmixture provided to an internal combustion engine. It is a furtherobject of the present invention to provide such a sensor, and anelectrical system responsive to the sensor, to produce a control signalwhich may be used to maintain a desired, known exhaust gas chemistry. Itis a still further object of the present invention to provide such asensor which has an approximately linear change in the logarithm ofresistance in response to changes in the exhaust gas chemistry, and inparticular to changes in the partial pressure of oxygen within theexhaust gas. More particularly still, it is an object of the presentinvention to provide a sensor suitable for positioning within theexhaust system of an internal combustion engine and having a sensingelement or member with an electrical characteristic which varies in acontrollable and predictable fashion in the presence of changes in theexhaust gas chemistry produced by variations in the air/fuel ratio of afuel-lean combustion mixture provided to the engine. It is also anobject of the present invention to provide an electrical system,including the sensor of the present invention, for controlling anoperational amplifier to produce an output signal which may be used tocontrol or modulate the air/fuel ratio of a combustion mixture.

SUMMARY OF THE PRESENT INVENTION

The present invention provides an exhaust gas sensor having, as itsactive element, a cobalt monoxide ceramic member arranged so that itselectrical resistance may vary in response to changes in the exhaust gaschemistry which variations may be measured electrically. The sensor isalso provided with means for generating heat in the vicinity of thecobalt monoxide element to provide for rapidly heating this element to atemperature in excess of its minimum operating temperature. The cobaltmonoxide active element of the present invention is provided in asubstantially cylindrical form having a pair of electrodes applied tothe axially opposite ends of the cylinder. A pair of electrical wires iscoupled to the electrodes for application of a controlled electricalcurrent from which a voltage signal indicative of variations in theresistance of the cobalt monoxide sensing element may be derived. Theelectroded ends of the cylindrical element are supported within acompatible ceramic housing material, such as alumina, which is providedwith a transverse aperture to assure a flow of exhaust gases in closeproximity to the surface of the sensing element. The heater wire isarranged in a helix wrapped about the housing structure and a covermember may be provided to protect the heater wire, the housing members,and the sensing element.

The present invention also provides an electrical circuit, including thecobalt monoxide ceramic sensing element of the present invention, forproviding a pair of inputs to an operational amplifier such that theoutput signal of the operational amplifier may indicate by its magnitudethe magnitude of any variation of the air/fuel ratio of the combustionmixture and, by its polarity, the quality of any variation in air/fuelratio. Quality is intended to mean whether the excursion of the air/fuelratio is an increase in fuel content or a decrease in fuel content fromthe fuel content at the selected air/fuel ratio.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of an internal combustion engineshowing the installation of the present invention in the exhaust systemthereof.

FIG. 2 is a diagrammatic view showing the sensor according to thepresent invention in section and including a block diagram of anelectrical system associated with the sensor for deriving a usefuloutput.

FIG. 3 is an electrical circuit illustrating one embodiment of the blockdiagram system shown in FIG. 2.

FIG. 4 is an exploded view of one embodiment of the sensor according tothe present invention.

FIG. 5 is a graph illustrating the electrical behavior of the presentinvention in terms of engine operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing wherein like numbers designate likestructure throughout the various views thereof, FIG. 1 is anillustration showing an internal combustion engine 10 having anassociated combustion mixture intake system 12 and an exhaust system 14.Intake system 12 is comprised of an intake manifold 16 fixedly coupledto the engine 10 and is arranged to provide a flow of the combustibleair/fuel mixture to the intake ports, not shown, of the engine 10. Anair/fuel mixture preparation device 18 is fixedly coupled to the intakemanifold 16 upstream from the intake ports of the engine 10 and may be,for example, a carburetor or any other form of device for producing anair/fuel mixture. An intake air cleaner 20 is illustrated as beingupstream from the mixture preparation device 18 and is of notoriouslywell known construction.

Air/fuel mixture preparation device 18 preferably includes an electricalmeans for modulating either the air or the fuel content of the air/fuelmixture in order to controllably vary the mixture ratio. The mixtureratio may be controllably varied electrically through the use of acontinuous flow fuel delivery device having an electrically controllablymetering orifice in conjunction with a carburetor or fuel injectionsystem or through a scheduling control arranged to modulate the injectorvalve open time in an intermittent injection fuel injection system.Alternatively, the air content of the combustion mixture may be variedby an electrically controlled air valve within intake system 12. It willbe appreciated that this brief description is intended to be merelyillustrative and that a large variety of means and mechanisms isavailable to accomplish the desired result of modulating either the airor the fuel content of the air/fuel mixture in accordance with anelectrical input signal derived as described hereinbelow with referenceto FIGS. 2 and 3.

Exhaust system 14 includes an exhaust manifold 22 and an exhaust conduit24. Exhaust conduit 24 may communicate with an exhaust gas reactor ofthe thermal or catalytic variety and may also communicate with anexhaust silencer such as a muffler in conducting the combustionby-product exhaust gases from the combustion chambers of the engine 10to the atmosphere while reducing the gaseous and noise pollutants whichare also by-products of the combustion process. An exhaust gas sensor 26according to the present invention is threadedly received by a wallportion of conduit 24 so that the active portion of the exhaust gassensor 26 extends into the interior region of conduit 24. First andsecond pairs of electrical leads 28, 30 extend from the exhaust gassensor 26 and communicate with electrical apparatus as describedherinbelow with reference to FIGS. 2 and 3. The precise placement ofsensor 26 will depend upon temperature and other considerations such asaccessibility, vibration and availability of electrical energy.

It will be appreciated that the various mechanical constituents of thissystem as hereinabove briefly described may be held in assembledrelationship by any suitable means such as the well known and presentlyemployed bolting of the various components together with the use ofgaskets between adjacent components whereever necessary to avoiduncontrolled leakage at these joints. As thus described, internalcombustion engine 10 with its air/fuel mixture intake system 12 and itsexhaust system 14 may be a reciprocating piston internal combustionengine or may also be the more recently developed rotary internalcombustion engine.

Referring now to FIGS. 2 and 4, the exhaust gas sensor 26 according tothe present invention is illustrated in section view with an electricalblock diagram (FIG. 2) and in an exploded view of the sensor per se(FIG. 4). The exhaust gas sensor 26 is comprised of a sensing element 32supported at its ends by housing members 34, 36. As illustrated in FIGS.2 and 4, sensing element 32 comprises a substantially cylindrical blockor unit of cobalt monoxide ceramic material which is adapted on itsaxial ends for receipt of the pair of electrical leads 28a, 28b. Forexample, the axially remote ends of the generally cylindrical sensingelement may be provided with an electrode forming coating of platinumpaste material and the ends of wires 28a, 28b, which in this instanceare preferably also platinum, are bonded to these surfaces. It will beappreciated that other sensing element geometrys and sensorconfigurations are contemplated. For example, the wafer geometry andconfiguration of the above-noted Stadler et al. U.S. Pat. No. 3,886,785is considered to be feasible and to produce results comparable withthose of the illustrated sensor 26.

In the illustrated embodiment, housing members 34, 36 are generallycylindrical members with housing member 36 sized to be received within asuitable axially extending bore provided within housing member 34.Housing member 36 is provided with a support recess arranged to receiveone axial end of the sensing element 32. Housing member 34 is providedwith an axially extending bore 38 which is sized to permit passage of,and receive, sensing element 32 upon insertion of sensing element 32 andhousing member 36 within housing member 34. Housing member 34 is alsoprovided with transverse bore 40 which is arranged to extend completelythrough housing member 34 and to be positioned to expose sensing element32, intermediate the ends thereof, to the gaseous environment of thesensor 26. Transverse bore 40 is therefore operative to provide forcommunication of a flow of exhaust gases around, and in close proximityto, sensing element 32. Housing member 34 is also provided with amounting flange 42 having a through passage 44. Hollow support member 46is received within through passage 44. One lead, 28a, of the first pairof electrical leads 28 is arranged to extend through the hollow supportmember 46 for electrical communication with one end of sensing element32 while the other electrical lead, 28b, of the first pair of electricalleads 28 communicates electrically with the other end of sensing element32.

Heating coil 48 is arranged to surround housing member 34. Each end ofheating coil 48 communicates with one lead of the second pair ofelectrical leads 30. This communication may be through flange portion 42through cover member 52, or through hollow support member 46. Theelectrical leads 30 are arranged to communicate with an electrical powersupply 50 for flow of a heating current through heating coil 48. aligned

Cover member 52, as shown in FIG. 4, is arranged to encapsulate heatercoil 48 and housing member 34. Cover member 52 therefore provides forprotection of the heating coil 48 and of the sensing element 32 topermit ease of handling of the sensor 26 during its installation in theexhaust system 14. Cover member 52 also proves for isolation of heatercoil 48 from the cooling effects of the major flow of the exhaust gasstream. Cover member 52 is provided with a pair of transversely alignedwindows 54 which are positioned to be in registry with the transversebore 40 when cover member 52 engages mounting flange portion 42. Thus,cover member 52 will not hinder or impede the flow of gases around thesensing element 32.

With particular reference now to FIG. 2, sensor 26 is shown in asectional view with cover member 52 removed. An electrical systemoperative with the sensor 26 of the present invention is alsoillustrated in a block diagram. The first pair of electrical leads 28and the power supply 50 communicate with resistance responsivecontroller means 56. Upon the application of electric power to heatingcoil 48, heating coil 48 will operate to elevate the temperature of thesensing element 32 to assist in rapid heating of sensing element 32 toits minimum operating temperature of approximately 900°C.

At temperatures below about 900°C and at partial pressures of oxygen inthe exhaust system 12 characteristic of operation of an internalcombustion engine in the lean regime, cobalt monoxide undergoes a phasechange to a form of oxide which does not exhibit the desired resistancevariations. Heating coil 48 enables rapid heating sufficient toencourage the phase change to the monoxide phase of any surface materialwhich may have converted during the previous cool-down. However, theexhaust system 14 of an internal combustion engine cools sufficientlyrapidly that the total mass of material converting to the desired oxidephase will be slight.

We have determined that the logarithm of the electrical resistance ofcobalt monoxide ceramic material varies in a generally linear fashion inresponse to changes in the partial pressure of oxygen in the exhaust gasby-products of lean regime combustion when the temperature of thesensing element is maintained above about 900°C. Furthermore, thisresistance change is repeatable, predictable and occurs with a responsetime of less than about 1 second. The region of partial pressures ofoxygen over which this resistance change occurs corresponds to thepartial pressures of oxygen predicted by chemical computation to occuras the exhaust gas by-products of combustion of a mixture having anair/fuel ratio in the lean regime.

We have determined that the electrical resistance of cobalt monoxideceramic is related to the partial pressure of oxygen by the expression

    R ∝ Po.sub.2.sup..sup.-1/4  e .sup..sup.+ E/(K T)

where R is the resistance in ohms, Po₂ is the partial pressure of oxygenin Atmospheres, e is the base of the natural logarithm, E is theactivation energy of the material in electron volts, T is thetemperature in degrees Kelvin and K is the Boltzmann constant inelectron volts per degree. As the partial pressure of oxygen drops belowabout 10.sup.⁻⁶ Atmosphere the power in the above expression changesgradually from -1/4 to -1/6 . For cobalt monoxide ceramic material, E isapproximately equal to one-half (0.5) of an electron volt and thedependence of the expression upon temperature can be seen to be slight.Thus, accurate temperature control above about 900°C is not required. Byproviding a known voltage potential between leads 28a, 28b of the firstpair of electrical leads 28 and by measuring the amount of currentflowing therethrough, the partial pressure of oxygen of the gaseousenvironment of sensing element 32 can be determined.

In order to maintain operation of internal combustion engine 10 at aselected air/fuel ratio within the lean regime, i.e., where uncombustedand therefore excess oxygen may exist within the exhaust gases producedas by-products of the combustion process, the partial pressure of oxygenof the exhaust gas stream must be maintained substantially constant at aselected value. By electrically monitoring resistance changes of thesensing element 32 variations in this partial pressure of oxygen may bedetected. By suitably operating upon detected changes in the resistancemeasurements, resistance responsive controller means 56 may be arrangedto provide an electrical signal, at output terminal 58, which may bemade representative of the amount of change necessary in either the air,or the fuel, content of the air/fuel mixture provided by mixturepreparation device 18 to maintain a selected partial pressure of oxygenwithin the exhaust system 14.

Referring now to FIG. 3, an electrical schematic of the resistanceresponsive controller means 56 of FIG. 2 is shown. The circuit of thisfigure also includes a representation of power supply 50 in the form ofa battery and incorporates the variable electrical resistance of sensingelement 32 as variable resistance 132. The circuit of FIG. 3 includeslimit resistor 60 and zener diode 62 which are cooperative to provide aregulated voltage across the conductors 64, 66. Reference voltagedivider 68 is comprised of a fixed resistance 70 and a variableresistance 72, such as a potentiometer, connected electrically in seriesbetween conductors 64, 66. Sensor voltage divider 74 is similarlycomprised of a fixed resistance 76 connected electrically in series withthe variable resistance 132 formed by sensing element 32. The referencevoltage junction 78, formed by the junction of fixed resistance 70 withvariable resistance 72, is communicated to one input terminal ofoperational amplifier 80. Sensor voltage junction 82, formed by thejunction of fixed resistance 76 with variable resistance 132, iscommunicated to the other input terminal of operational amplifier 80.Operational amplifier 80 may be, for example, a type μ 741 operationalamplifier. This type of operational amplifier is available through alarge number of sources under the identifying number μ 741.

The variable resistance 72 in the reference voltage divider portion ofthe network of FIG. 3 may be calibrated to provide a voltage atreference junction 78 which is exactly equal to the voltage appearing atsensor voltage junction 82 when the partial pressure of oxygen in theatmosphere in which sensor 26 is immersed is exactly equal to thatcorresponding to operation of the internal combustion engine with thecombustion mixture at the desired air/fuel ratio. By arrangingoperational amplifier 80 to produce at its output terminal 58 a voltagesignal which is a selected multiple of the difference between thevoltage values appearing on its input terminals, operation of theinternal combustion engine 10 at the precisely desired air/fuel ratiowill result in a zero voltage signal appearing on the output terminal58. Any excursion in the air/fuel ratio of the combustion mixture awayfrom the desired value will result in a shift in the voltage valueappearing at sensor voltage junction 82 which, when compared with thevoltage appearing at reference voltage junction 78, will result in anoutput signal appearing at terminal 58. Thus, the signal appearing atoutput terminal 58, including a zero signal, will indicate theelectrical resistive value of sensing element 32 relative to apreselected value and hence the partial pressure of oxygen in theenvironment of the sensor 26. The magnitude of the signal appearing atterminal 58 will therefore be indicative of the magnitude of thevariation of the air/fuel ratio from the desired value while thepolarity of the signal appearing at output terminal 58 will beindicative of the nature or quality of the excursion. For example, apositive polarity signal appearing at junction 58 may indicate that theair content of the air/fuel mixture is excessive resulting in anincrease in the air/fuel ratio while a negative polarity signalappearing at output terminal 58 may indicate that the air content of theair fuel mixture is inadequate producing a decrease in the air/fuelratio of the combustion mixture. The magnitude and the polarity of theoutput signal generated by operational amplifier 80 may be readilytailored to be directly compatible with the particular modulation deviceselected for inclusion in intake system 12 such that the polarity andthe magnitude of signal appearing at output terminal 58 willautomatically command the proper corrective measures to maintain theair/fuel ratio of the combustion mixture at the selected value. It willbe appreciated that the specific electrical network illustrated in FIG.3 is representative only and that other electrical networks may also beutilized with the present invention to achieve beneficial results. Forexample, a network comparable to that illustrated in FIG. 3 of theabove-noted Kushide et al U.S. Pat. No. 3,868,846 may be utilized.

Referring now to FIG. 5, a graph is shown illustrating, by curve 84, therelationship of the partial pressure of oxygen present in volume percentin the exhaust gases produced by combustion of various air/fuel ratiocombustion mixtures. FIG. 5 also includes a graph illustrating, by curve86, typical resistance value for sensing element 32 at the variousillustrated air-to-fuel ratios. It will be appreciated that the ohmicvalues given for various resistances are illustrative only and thatspecific resistance values will depend upon the particular geometry anddimensions employed for sensing element 32. However, the illustrateddifferential resistance, in percent, is typical. For example, theresistance of the cobalt monoxide ceramic sensing element 32 is larger,by approximately 70 percent, at an air/fuel ratio of 15 than it is at anair/fuel ratio of 20. Curve 86 is linear if graphed on a logarithmicscale.

Thus it can be seen that the present invention readily accomplishes itsstated objectives. By utilizing a cobalt monoxide ceramic sensingelement, fabricated from commercially available forms of pure cobaltmonoxide according to any of the well known ceramic formationtechniques, a sensor which demonstrates a repeatable and uniquelydefined resistance value for differing partial pressures of oxygenresults. By supporting the ends of a cylinder of cobalt monoxide ceramicsensor material with a compatible ceramic material such as alumina theproperties of the cobalt monoxide ceramic do not demonstrate any longterm changes which would effect the electrical characteristics of thecobalt monoxide ceramic material and the sensing element is wellsupported against vibration. By utilizing a surrounding heating coil 48,the cobalt monoxide ceramic material may be brought rapidly to itsoperating temperature so that the sensor 26 may become functional in ashorter amount of time from an internal combustion engine startup andmay be maintained above 900°C during operation so that the monoxidephase of the cobalt oxide ceramic may be maintained. Use of the covermember 52 facilitates handling and provides for protection of the devicewhen being handled while providing insulation against thermal shock andany cooling of the sensing element 32 or the heating coil 48 caused bythe absorption of heat by the flow of the exhaust gas stream.

We claim:
 1. A sensor for sensing the partial pressure of oxygen gaswithin a gas conduit system for exhausting the gaseous combustionby-products of fuel-lean combustion from a combustion chamber of anengine, comprising in combination:a cobalt monoxide ceramic sensingelement having an electrical resistive path; electrically conductivemeans attached to said sensing element at opposite ends of saidelectrical path, operative to provide a flow of electric current throughsaid sensing element generally along said electrical path; housing meansfor receiving and supporting said sensing element, said housing meansincluding means for attachment to a wall portion of the gas conduitsystem and being arranged to expose a substantial portion of the surfaceof said sensing element to the gaseous combustion by-products within thegas conduit system; and electrical means communicating with saidconductive means for generating an electrical signal having a magnitudeand polarity indicative of the electrical resistance value of saidsensing element along said electrical path whereby the partial pressureof oxygen within the gaseous combustion by-products and hence theair/fuel ratio of the combustion mixture may be defined.
 2. The sensorof claim 1 wherein said electrical means comprise:reference means forgenerating a reference voltage value indicative of a desired air/fuelratio for the combustion mixture; and comparison means for comparingsaid reference value with the value indicative of the electricalresistance of the sensing element, said comparison means operative togenerate said electrical signal whereby said generated electrical signalis also indicative of the magnitude and quality of the excursion of theair/fuel ratio from the desired air/fuel ratio.
 3. The sensor of claim 1including cover means attached to said housing means and arranged tosurround and protect said housing means and said sensing element.
 4. Thesensor of claim 3 wherein said cover means include means defining atleast two apertures arranged to provide a flow of the gaseous combustionby-products over the surface of at least a portion of said sensingelement.
 5. The sensor of claim 1 including heating means, auxiliary tothe combustion by-products, for heating said sensing element to atemperature above about 900°C.
 6. In a system for monitoring theair/fuel ratio of the combustion mixture provided to an internalcombustion engine intended to operate under steady state conditions at apreselected air/fuel ratio wherein an exhaust gas sensor is situatedwithin the stream of exhaust gases produced by the engine and isarranged to produce an electrical signal indicative of the partialpressure of oxygen in the exhaust gases and including electrical meanscommunicating with and responsive to the sensor and arranged to generatean electrical signal indicative of the air/fuel ratio of the combustionmixture, the improvement wherein the exhaust gas sensor is comprised ofa cobalt monoxide ceramic sensing element connected electrically inseries with a source of electrical energy and the electrical means arearranged to respond to the current flow through said cobalt monoxidesensing element to generate an output signal having a magnitude andpolarity indicative of excursion of the air/fuel ratio of the combustionmixture from a selected value in the lean regime.
 7. The system of claim6 wherein said electrical means comprise:reference means for generatinga reference voltage value indicative of a desired air/fuel ratio for thecombustion mixture; and comparison means for comparing said referencevalue with a value indicative of the electrical resistance of thesensing element, said comparison means operative to generate saidelectrical signal, whereby said generated electrical signal is alsoindicative of the magnitude and quality of the excursion of the air/fuelratio from the desired air/fuel ratio.
 8. The system of claim 6including cover means attached to said housing means and arranged tosurround and protect said housing means and said sensing element.
 9. Thesystem of claim 8 wherein said cover means include means defining atleast two apertures arranged to provide a flow of the gaseous combustionby-products over the surface of at least a portion of said sensingelement.
 10. The system of claim 6 including heating means, auxiliary tothe exhaust gases, for maintaining the temperature of the cobaltmonoxide above about 900°C in operation.